From: Joe Chang (changjoe@earthlink.net)
Date: Mon Mar 03 2003 - 17:01:00 GMT-3
----- Original Message -----
From: "roi hermoni" <roih@012.net.il>
To: <ccielab@groupstudy.com>
Sent: Monday, March 03, 2003 12:25 PM
Subject: BGP case.
> Hi All,
>
>
>
> Look at this scenario:
>
>
>
> networkA -------->
>
> networkB -------->R1 -------->R2 ----->internet (normal flow)
>
> -------->R3 ----->internet
>
>
>
>
>
> Networks A and B are connected to R1 and have route to R2 and then
Internet.
>
> There are cases where I want that networkA will take different path, means
via
> R3.
>
> I want that R3 will do the new advertisement.
>
> Some minor configs on R1 are possible but the main config/advertisement R3
> will do.
>
> In your answer please forget PBR (no ip policy on interfaces but route-map
are
> o.k).
>
> I'm searching for BGP tricks to do it.(tags are also possible)
>
> The main problem here that legacy routing works on destination (which in
my
> case is unknown)
>
I see that your problem is that there may be destinations on network B that
net A would need to reach or Internet routes that only R2 can provide.
Because of this you cannot simply policy route all ingress packets from
network A to R3.
> And here I have to decide based on the source without PBR.
>
> Is it possible
>
I can't think of any solution only on R1, BGP-feature or otherwise, that is
not error-prone or will flat-out fail.
You might want to try this solution:
1)Advertise network B from R1 to R3. Also advertise internet routes that R2
knows but R3 does not. Use the no-advertise community to prevent R3 from
advertising this information back out the Internet.
2)Create a tunnel between R1 and R3. Use PBR to route all traffic from net A
to R3.
3)On R3, establish a default route out the Internet link.
If R3's Internet link leads out to net A's destination, then the packet will
be routed out that link.
If R3's Internet link does not provide a path, and R1 or R2 does, R3 will
send the packet back to R1.
If R3 does not have any information about the destination, the default route
will prevent a routing loop from occurring.
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